Transmitting supervisory messages from a fiber amplifier

ABSTRACT

The present invention is based on the idea that an optical band-pass filter (22) is employed in addition to its normal function for modulating a signal passing through the fiber amplifier on such a low frequency and amplitude that this modulation does not disturb the actual data transmission. This is realized in such a manner that the control signal of the band-pass filter (22), which is used in prior art fiber amplifiers only for optimizing the signal-to-noise ratio, for example, is used in the amplifier of the present invention for modulating the output signal of the fiber amplifier at the rate of a supervisory message.

BACKGROUND OF THE INVENTION

The invention relates to data transmission between such amplifiers thatuse optical fiber as their data transmission channel.

A fiber amplifier is a device with which an optical signal can beamplified without converting the signal into an electrical signal inbetween. These amplifiers are used with very high transmission rates. Atypical value of transmission rate may be 1 GHz, for example. Theoperation of a prior art fiber amplifier is illustrated in FIG. 1. Inthe figure the conductors transmitting optical signals are indicatedwith double lines and the conductors transmitting electrical signals areindicated with single lines. An input signal 1 of the amplifier isconducted via an optical isolator 12 to the second input of a wavelengthcombiner 14. The function of the optical isolator 12 is to attenuatereflections, for example. An input signal detector 16 after the opticalisolator 12 detects the level of the input signal and generates anelectrical signal corresponding to this level. A pump laser unit 18supplies the second input of the wavelength combiner 14. Theamplification of a signal operates in the following way. The pump laser18 produces light whose wavelength is e.g. 980 or 1480 nm, whereas thewavelength of the signal light is 1,550 nm, for example. The photons ofthe pump laser 18 are conduced to an erbium-doped fiber 10 where theyexcite erbium atoms of the fiber 20. Some of the erbium atoms return tothe ground state via spontaneous emission. When the photons of thesignal light is directed to the erbium atom tuned by the photons of thepump laser, the erbium atom emits a photon corresponding to the photonof the signal light. The erbium-doped fiber 20 is followed by aband-pass filter 22 passing through the photons displaced by the photonsof the signal light, but prevents the travel of the photons which aregenerated mostly by their spontaneous emission and whose wavelength isnot similar to the wavelength of the signal light. An output signal 2 ofthe band-pass filter is an amplified optical signal transmitted to a(not shown) receiving station. Fiber amplifiers are discussed e.g. inOptical amplifiers and their Applications, edited by S. Shimada and H.Ishido, ©John Wiley & Sons, 1992.

Signal wavelength and characteristics of the band-pass filter 22 varye.g. according to temperature or change considerably if some componentof the system has to be replaced. In that case, the amplifier should beable to be adjust the pass wavelength of the band-pass filter byexternal, e.g. electrical control. Prior art arrangements comprise acontroller 26 for this purpose. The controller 26 monitors the detector16 of the input signal and a detector 24 of the output signal andamplifies the control signal of the band-pass filter 22 for maximizingthe amplification of the amplifier or the correlation between the outputand input signals.

As a fiber amplifier normally only amplifies a signal with a certainfactor, without adding information to the signal, signalling associatedwith supervision and control of the amplifier generally has to becarried out with separate systems. This is a particular problem whenusing a so-called intermediate amplifier which is far (e.g. over 100 km)from both the transmitter and the receiver.

European Patent Application 415 438 discloses an art where supervisorymessages are transmitted by modulating the control signal of a pumplaser unit in such a manner that the excess amount of pumping light fromthe amplifier contains a supervisory signal to be transmitted. Thisprior art solution has, however, several limitations. First, the priorart solution is only applicable on a wavelength of 1,480 nm as theexcess pumping light on a wavelength of 980 nm will be absorbed on along data link by an information transmitting fiber. At the moment, itseems that the use of the wavelength of 1,480 nm is being abandoned asthe wavelength of 980 nm produces better noise characteristics to thefiber amplifier and the total efficiency ratio of the amplifier will bebetter. Secondly, the prior art solution requires two separate opticalsystems, one for receiving a payload signal and one for receiving asupervisory signal. The reception of supervisory signals according tothe prior art requires a separate WDM component (Wavelength DivisionMultiplexer). Thirdly, the prior art solution is only useable on suchfrequencies whose period is shorter than the lifetime of thefluorescence state.

BRIEF SUMMARY OF THE INVENTION

The object of the invention is thus to accomplish signalling related tothe supervision of a fiber amplifier in such a manner that the problemsassociated with prior art can be solved. The object of the invention isachieved with the methods and systems which are characterized by what isstated in the independent claims.

The invention is based on the idea that the optical band-pass filter 22is employed in addition to its normal function for modulating a signalpassing through the fiber amplifier on such a low frequency andamplitude that this modulation does not disturb the actual datatransmission. This is realized in such a manner that the control signalof the band-pass filter 22, which is used in prior art fiber amplifiersonly for optimizing the signal-noise ratio, for example, is used in theamplifier of the invention for modulating the output signal of the fiberamplifier in the rate of the supervisory message.

A primary advantage of the method and system of the invention is that noseparate signalling has to be arranged for supervising the amplifier.Another advantage is that changes required for the amplifier forrealizing the invention are small. For example, the same opticalcomponents used for receiving a payload signal can be employed forreceiving supervisory messages.

BRIEF DESCRIPTION OF THE SEVERAL VIEW OF THE DRAWING

The invention will now be explained in more detail by means of preferredembodiments with reference to the accompanying drawings, where

FIG. 1 shows a block diagram of a fiber amplifier of prior art;

FIG. 2 shows a fiber amplifier completed in accordance with a basicembodiment of the invention;

FIG. 3 shows an amplifier station comprising two fiber amplifierscompleted in accordance with the invention;

FIG. 4 shows amplitude modulation of an output signal of the fiberamplifier; and

FIG. 5 shows one method for detecting supervisory messages.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 shows a fiber amplifier completed in accordance with a basicembodiment of the invention. All parts of FIG. 1 are also included inthe embodiment shown in FIG. 2 although FIG. 2 only shows of them thecontroller 26 and the band-pass filter 22. In accordance with theinvention, the controller 26 is controlled by a processor 30 whosestructure can be a general-purpose computer, or a controller or the likeintegrated into one printed board or even into one chip. FIG. 2 isassumed to comprise all the accessories required by the processor, suchas a system clock, bus buffers, etc. Of these accessories FIG. 2 showsonly a read/write memory 34 and a program memory 36. The program memory36 can be any memory in which the program is maintained during powerfailures, e.g. read only memory (ROM), flash memory or a hard disk. FIG.2 shows a solution where the processor is connected to the controller 26and the memories 34 to 36 via a system bus 32. A block 40 indicated inFIG. 2 comprises all the information the amplifier is to forward assupervisory messages. The two-way arrows connected to the system bus 32describe an arrangement where the processor 30 has in use all theinformation of the controller 26 and the processor 30 may have an effecton the operation of the controller 26 and through it, on the operationof the band-pass filter 22. Similarly, information on thecharacteristics of the band-pass filter can be stored in the read/writememory 34 which can be accessed both from the processor 30 and thecontroller 26.

The block 40 comprises all the information that the amplifier is toforward as supervisory messages. This information may containinformation on the opera-tion of the fiber amplifier, on conditions, thevideo signal obtained from a surveillance camera, etc. It is notessential for the invention what the supervisory messages contain buthow the messages are transmitted.

In accordance with the invention, the processor 30 controls thecontroller 26 via the system bus 32 in such a manner that the controller26 adjusts the control signal of the band-pass filter 22 for modulatingthe optical signal passing through the fiber amplifier on a lowfrequency and amplitude. A low amplitude here denotes that themodulation level is typically a couple of percentages and a lowfrequency denotes a frequency that is considerably lower than thefrequency used by the fiber amplifier for the actual traffic.

When using a band-pass filter 22 for modulating an optical signal,particular attention should be paid to the accuracy of a control signal22s. There are at least two reasons for this. First, the way theband-pass filter 22 allows an information transmitting optical signal topass through is not a monotonous function for the control signal 22s,but this function has its peak at a specific value of the controlsignal. Secondly, the spontaneous emission explained in FIG. 1, which isnoise for the receiver, is within the same band as the wavelength of thesignal light.

The following shows one arrangement for calibrating amplitude modulationin a fiber amplifier of FIG. 2. It is assumed that the fiber amplifiertransmits and amplifies the optical signal continuously. (i) The controlsignal 22s of the band-pass filter is set to its minimum value. (ii) Thecontrol signal 22s of the band-pass filter is magnified in small stepsuntil the ratio of the output signal of the fiber amplifier to the inputsignal 16s reaches its maximum point and the value of this controlsignal 22s is stored in the read/write memory (RAM) 34. This value ofthe control signal 22s is indicated by S₀. (iii) The value of thecontrol signal 22s is altered until the ratio of the output signal ofthe fiber amplifier to the input signal 16s is reduced from the peakvalue measured in step (ii) to a desired modulation level, for example 2to 3%, and the obtained value of the control signal 22s is stored in theRAM 34. This value of the control signal 22s is indicated by S₁. Whenthe value of the control signal 22s is altered in step (iii), the onethat best attenuates the power of spontaneous emission is to be selectedas the direction (up/down) of the alteration.

It was assumed above that the fiber amplifier transmits an opticalsignal continuously. If the optical conductor passing through the fiberamplifier does not transmit a payload signal, it can transmit nullcharacters. If these null characters are not used, the calibrationprogram of the processor is to be contain a step where it is detectedthat the optical signal is not present at all and then calibration isnot attempted.

When the equipment of the invention is formed in the fiber amplifier fortransmitting supervisory messages, this equipment can be used foradjusting the band-pass filter 22 essential even in the normal operationof the fiber amplifier. The calibration program stored in the programmemory 36 can be made responsive to condition information, especially totemperature information, obtainable from a data acquisition block 40. Itdepends on the characteristics and the expected installation site of theband-pass filter if condition information is to be supplemented by airpressure and/or relative humidity. Also, the calibration program storedin the program memory 36 can be activated by an external commandtransmitted via the optical fiber. In that case, the fiber amplifier isto be connected to a detector to be described below in FIG. 5 and also,the signals have to be assembled into frames and in this case, the fiberamplifier whose band-pass filter is to be calibrated is to be indicatedas the receiver.

Calibration can alternatively be carried out continuously in such amanner that calibration is performed for example once every minute orwhenever the fiber amplifier is not transmitting supervisory messages.Calibration does not prevent or disturb the normal operation of thefiber amplifier. Then null characters, such as frames whose number ofdata bytes is zero, have to be transmitted for calibration.

The other parts of the fiber amplifier of the invention are consideredto be known to persons skilled in the art. The critical component is theelectrically controllable band-pass filter 22 whose essential parametersfor the invention are stability and settling time. Suitable band-passfilters are manufactured e.g. by Queensgate Instruments Ltd inBerkshire, England.

When the band-pass filter 22 is calibrated as described above, it can beused for modulating an optical signal in the following way, whereinreference is made to FIG. 4. For the sake of simplicity, it is assumedthat supervisory messages are transmitted in binary 0 and 1 states, inwhich case the presence of amplitude modulation corresponds to state 1and its absence to state 0. Time T corresponding to one bit of asupervisory message is divided into four periods T1 to T4. A binary 1state pulse is transmitted in the following way:

period T1: the value of the control signal 22s is S0,

period T2: the value of the control signal 22s is S1,

period T3: the value of the control signal 22s is S0.

period T4: the value of the control signal 22s is S1.

Binary 0 state pulse is transmitted otherwise in the same way as the 1state pulse but in period T2 the value of the control signal 22s is alsoS0, that is, the amplitude is the same as in periods T1 and T3. Itshould be noted that the frequency of the optical signal in FIG. 4 isdrawn considerably smaller than in reality in relation to the frequencyof amplitude modulation. In reality thousands or millions of opticalsignal 2 periods fit into one amplitude modulation sequence.Furthermore, the modulation level is drawn considerably larger than theoptimal value for the sake of illustration.

The above modulation method explained in connection with FIG. 4 is knownper se and it is called Frequency Shift Keying (FSK). In short, in theFSK clock pulses are continuously sent at specific intervals (in periodsT4). If there is a data pulse between two clock pulses (in period T2),this is interpreted as binary 1, otherwise as binary 0. An advantage ofthe FSK is that it is simple to implement, but a disadvantage is itsslow transmission rate as each 1 state data requires four changes to theamplitude modulation state. It is also evident that on the basis of theabove explanation, the invention can also be used with more advancedmodulation methods.

FIG. 5 shows as a block diagram how supervisory messages can beindicated in a receiving station, of which other parts are not shown.The output signal 2 of the fiber amplifier according to FIG. 2 isreceived at a receiving station. The optical signal is processed afteran optical isolator located there for normal telecommunication in anordinary way. For detecting supervisory messages of the invention, theoptical signal is converted into an electrical signal by a detector 104.The signal of the detector 104 is conducted to a transimpedanceamplifier 106. This utilizes the fact that the frequency used by thefiber amplifier for the actual traffic is approximately 100,000 timesgreater than the frequency of supervisory messages. The detector 104and/or transimpedance amplifier 106 realized with ordinary componentsallow only the frequency band of supervisory message (typically sometens of kilohertz) to pass through. A DC component is removed from theinput signal of the transimpedance amplifier 106 by a capacitor 108after which the signal is coupled to an automatically adjustableamplifier 110 and therefrom to a threshold value detector 112. Theoutput signal of the threshold value detector 112 is "1" when a pulse ispresent in the signal transmitting the supervisory message and otherwise"0". The output signal of the detector 112 can be converted in paralleleither by such series-to-parallel converters that are used in modems anddisk drives or, alternatively, by a computer program.

The circuit of FIG. 2 can transmit supervisory messages to the receiverof an optical signal but not to its sender. FIG. 3 shows how thearrangement of the invention is applied to the two-way transmission ofsupervisory messages. In view of FIG. 2, the circuit of FIG. 3 issupplemented by another fiber amplifier of FIG. 1 for operation in theopposite direction. Operation in one way is called outgoing directionand in the other way return direction. The components in the outgoingdirection in FIG. 3 are similar to those in FIG. 2. The components inthe return direction are indicated with apostrophes. The supervisorymessages to be returned are in this case obtained from the detector 16'of the input signal in the return direction, which is not shownseparately.

In the transmission of supervisory messages of fiber amplifiers of theinvention, special steps should be taken to ensure that the informationto be transmitted is error-free. There are several reasons for this; forexample, the fact that especially fiber amplifiers used as intermediateamplifiers can be a long distance from the preceding or next stations,whereby an unnecessary maintenance call caused by an erroneous messagecan be very costly. Another reason is that a compromise has to be madebetween noise-free messages and the actual operation of the amplifier.The error-free transmission of supervisory messages is improved bymagnifying the modulation level. On the other hand, a high modulationlevel causes problems to the actual operation of the fiber amplifier. Anadvantageous compromise is brought about in such a manner that themodulation level used in transmitting supervisory messages is relativelylow, less than 10% and an effective error-correcting code is used fortransmitting supervisory messages. The excess information required byerror correction is not generally a problem as the information contentof supervisory messages is generally small. An exception to this is asituation where the transmission of supervisory messages of theinvention is used e.g. for transmitting a video signal of a surveillancecamera. It is generally possible to transmit signals of this kindwithout error correction.

When there is only one intermediate amplifier between a transmitter anda receiver, it is clear that supervisory messages are sent from thisintermediate amplifier. If there are several successive intermediateamplifiers, the low modulation level of supervisory messages candisappear under the great amplification of the next intermediateamplifier. In that case, the safest way to operate is to assemble thesupervisory messages into frames containing the identifiers of thesender and the receiver of the message. These frames are detected in thenext intermediate amplifier and they are forwarded, as well as thespecified signals of the next intermediate amplifier, whereby theoriginal sender of the message is naturally set as the sender of theframe. In this case, the intermediate amplifier operates in the case ofsupervisory messages in the way that is largely analogous to bridgesused in local area networks. The frames of supervisory messages can alsobe simplified in such a manner that the receiver is not specificallyindicated by the receiver is always the last station in the chain.

An embodiment of the invention was explained in connection with FIGS. 2and 3 where a program for transmitting supervisory messages is stored ina program memory 36. It is also possible, of course, to store in theprogram memory 36 a certain basic software that can be converted andcomplemented by means of data transmitted as supervisory messages. Inthat case, a detector explained in connection with FIG. 5 should beadded to the fiber amplifier for the received supervisory messages. Theprogram memory 36 should be complemented so that the fiber amplifierdetects e.g. from the header fields of the frame that the receiver isthe fiber amplifier in question and its program memory. The alteredparameters or the parts of the program are stored in the read/writememory 34 and they replace the corresponding parts in the program memory36.

It is evident for those skilled in the art that as the art develops, thebasic idea of the invention can be realized in many ways. The inventionand its embodiments are thus not restricted to the examples describedabove, but they can vary within the scope of the claims.

What is claimed is:
 1. A method for transmitting a supervisory messagefrom a fiber amplifier transmitting an optical signal, the fiberamplifier comprising a band-pass filter responsive to an externalcontrol signal, the method comprising the steps of:determiningcalibration information indicating optical pass characteristics of theband-pass filter in response to the external control signal of theband-pass filter; forming a modulation signal corresponding to thesupervisory message and the calibration information; combining themodulation signal and the external control signal of the band-passfilter to form a combined control signal; and amplitude modulating theoptical signal of the fiber amplifier based on the combined controlsignal.
 2. A method according to claim 1, the calibration information isdetermined at predetermined time intervals.
 3. A method according toclaim 1, wherein the modulation level is in the range of 2 to 10%.
 4. Amethod according to claim 1, wherein the supervisory message is encodedinto a digital form.
 5. A method according to claim 4, wherein errordetection coding is used in the digital coding of the supervisorymessage.
 6. A method according to claim 4, wherein error correctingcoding is used in the digital coding of a supervisory message.
 7. Amethod according to claim 4, wherein the supervisory message isassembled into frames comprising an identifier of a sender.
 8. A methodaccording to claim 7, further comprising forwarding a received and codedsupervisory message, the forwarding step comprising:receiving thesupervisory message and defining the identifier of an original sendercontained therein; and re-transmitting the supervisory message, suchthat the identifier of the sender is the identifier of the originalsender of the supervisory message.
 9. A method according to claim 8,wherein the supervisory message is intended for a receiver and theframes also contain the identifier of the receiver.
 10. A methodaccording to claim 7, wherein the supervisory message is intended for areceiver and the frames also contain the identifier of the receiver. 11.A method according to claim 1 in an amplifier station comprising twofiber amplifiers, one for an outgoing direction and the other for areturn direction, wherein the control signal of the band-pass filter forthe return direction is controlled in the modulating step.
 12. A methodaccording to claim 1 wherein the fiber amplifier has a signal input atwhich the optical signal has an input signal value, the band-pass filterhas a signal output at which the optical signal has an output signalvalue, the ratio of the output signal value to the input signal valuehas a maximum value when the band-pass filter is controlled by a controlsignal having a given value, and the combined control signal has onevalue which causes the ratio to have a value less than the maximumvalue.
 13. A system for transmitting a supervisory message from a fiberamplifier transmitting an optical signal, the fiber amplifier comprisinga band-pass filter responsive to an external control signal, the systemcomprising:memory means for storing program code, supervisory messagesand calibration information indicating optical pass characteristics ofthe band-pass filter in response to the control signal of the band-passfilter; means for defining a supervisory message and for forming amodulation signal corresponding to the supervisory message and thecalibration information; and means for combining the modulation signaland the external control signal of the band-pass filter to form acombined control signal and for amplitude-modulating the optical signalof the fiber amplifier based on the combined control signal.
 14. Asystem according to claim 13, further comprising means for controllingthe band-pass filter in a return direction.
 15. A system according toclaim 13, wherein the memory means comprise a read-only memory forstoring the program code and a read/write memory for storing thecalibration information.
 16. A system according to claim 13, furthercomprising means for receiving a supervisory message.
 17. A systemaccording to claim 14 wherein the fiber amplifier has a signal input atwhich the optical signal has an input signal value, the band-pass filterhas a signal output at which the optical signal has an output signalvalue, the ratio of the output signal value to the input signal valuehas a maximum value when the band-pass filter is controlled by a controlsignal having a given value, and the combined control signal has onevalue which causes the ratio to have a value less than the maximumvalue.
 18. A system for receiving supervisory messages in a firstfrequency band superimposed on an optical telecommunication signal of afiber amplifier in a second frequency band, the system comprising:firstmeans for converting an optical signal of the fiber amplifier into anelectrical signal; a transimpedance amplifier operationally connected tothe first means, for separating a supervisory message from atelecommunication signal, the transimpedance amplifier having componentvalues such that the transimpedance amplifier only passes the firstfrequency band but not the second frequency band; and second meansoperationally connected to the transimpedance amplifier, for convertingthe supervisory message into a digital form.